The present invention relates to current transformers and, more specifically, to current transformers for use in circuit breakers.
Conventional circuit breaker devices with electronic trip units typically include a current transformer disposed around a line conductor of a distribution system providing electrical power to a load. The current transformer has a multi-turn secondary winding electrically connected to the circuit breaker""s electronic trip unit. The secondary winding is used to sense a current overload or imbalance in the aforesaid line conductors and, in response thereto, provide an output signal proportional to the current overload or imbalance to the trip unit. Upon receipt of such a signal the trip unit initiates an interruption of the current supplied to the load through the line conductors. The secondary winding may also be used to provide operating power to the electronic components within the circuit breaker""s electronic trip unit.
Operationally, the load current in a circuit breaker can cover a very wide range. Unfortunately, the magnetic materials commonly available for the core of the current transformer limit the dynamic range of the sensing device. Peak flux density is a limiting factor at the upper end of the dynamic range, while core loss/declining permeability is a limit at the lower end. For a given core material and required accuracy, these parameters limit the operating range of the current transformer. While the dynamic range could be extended by increasing the volume of the core material and/or the turns of a secondary winding, these solutions increase the size of the current transformer, which is often critical.
Often, a toroidal current transformer having a core in the shape of a toroid is utilized. A continuous, toroidal core provides a desirable, full dynamic range. However, the use of this type of core in a current transformer for use with a trip unit is not ideal. A trip unit is required to power-up and trip on the first half cycle. Therefore, it is necessary for the current output by the current transformer to have a uniform-sized first half cycle. In other words, it is necessary to employ a current transformer that outputs current with minimal attenuation. While a current transformer having a continuous, toroidal core would provide the desirable, full dynamic range of operating currents, such a current transformer would also provide an undesirable and significant remanence attenuation. Remanence is the flux density that remains in the core after the magnetizing force has ceased. Because of the significant remanence attenuation associated with a continuous, toroidal core, the use of a current transformer having such a core is less than ideal.
To reduce the level of remanence, an air gap can be added to the magnetic core by removing a section of the magnetic core, thus creating a xe2x80x9cCxe2x80x9d shaped core. When this is done, however, the air gap decreases the level at which saturation of the core takes place and thus reduces the range of current in which the current transformer can operate.
Another commonly used current transformer has a core made of stacked laminations. To prevent the core from becoming saturated at higher current levels, expensive magnetic steel laminates are used. These laminates are sized to allow short-circuit current sensing without causing the core to saturate. A current transformer having a stacked, laminated core transmits very little remanence attenuation, but their use is not ideal because they have a limited range of operation.
In an exemplary embodiment of the invention, a current transformer used to sense electrical current and provide operating power to an electronic trip unit includes a metal core having a top surface and a bottom surface, where the difference between the top and bottom surfaces defines a height of the core. The core has a concentric opening extending through the height so that the planar distance between an outside point on the concentric opening and the closest outside point of the core defines a thickness of the core at that point. A primary winding passes through the opening. A secondary winding also extends through the opening and encircles the thickness of the core. A partial air gap is located in the metal core.
This construction has a number of advantages over the prior art. The use of the air gap reduces the attenuation while still maintaining a maximum operating range. The size of the partial air gap can be pre-determined to optimize the current transformer functionality by minimizing the remanence attenuation while at the same time maximizing the current operating range.